Cooking and grinding reduces the cost of meat digestion

Comparative Biochemistry and Physiology, Part A 148 (2007) 651 – 656
www.elsevier.com/locate/cbpa
Cooking and grinding reduces the cost of meat digestion
Scott M. Boback a, Christian L. Cox a, Brian D. Ott a, Rachel Carmody b,
Richard W. Wrangham b, Stephen M. Secor a,?
a Department of Biological Sciences, University of Alabama, Tuscaloosa, AL 35487, USA
b Department of Anthropology, Harvard University, Peabody Museum, 11 Divinity Avenue, Cambridge, MA 02138, USA
Received 7 June 2007; received in revised form 8 August 2007; accepted 9 August 2007
Available online 16 August 2007
Abstract
The cooking of food is hypothesized to have played a major role in human evolution partly by providing an increase in net energy gain. For meat,
cooking compromises the structural integrity of the tissue by gelatinizing the collagen. Hence, cooked meat should take less effort to digest compared
to raw meat. Likewise, less energy would be expended digesting ground meat compared to intact meat. We tested these hypotheses by assessing how
the cooking and/or grinding of meat influences the energy expended on its digestion, absorption, and assimilation (i.e., specific dynamic action, SDA)
using the Burmese python, Python molurus. Pythons were fed one of four experimental diets each weighing 25% of the snake's body mass: intact raw
beef, intact cooked beef, ground raw beef, and ground cooked beef. We measured oxygen consumption rates of snakes prior to and up to 14 days
following feeding and calculated SDA from the extra oxygen consumed above standard metabolic rate. Postprandial peak in oxygen consumption,
the scope of peak rates, and SDA varied significantly among meal treatments. Pythons digesting raw or intact meals exhibited significantly larger
postprandial metabolic responses than snakes digesting the cooked ground meals. We found cooking to decrease SDA by 12.7%, grinding to decrease
SDA by 12.4%, and the combination of the two (cooking and grinding) to have an additive effect, decreasing SDA by 23.4%. These results support
the hypothesis that the consumption of cooked meat provides an energetic benefit over the consumption of raw meat.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Cooking; Grinding; Meat; Pythons; Specific dynamic action
1. Introduction
include a reduction in gut size and an increase in daily energy
expenditure. In support of the energetic significance of cooking,
Food scientists typically advance three main reasons for why
individuals that consume only raw food tend to be thin with little
meat is cooked; improved palatability, preservation, and protec-
stored energy, with such women possessing impaired reproductive
tion from pathogens and parasites (Charley, 1982; Lawrie, 1991;
function (Koebnick et al., 1999). Also, there is ample information
Barham, 2000; Warriss, 2000). By contrast, the consequences of
showing that cooking increases the digestibility of starch and the
cooking for net energy availability have conventionally been
glycemic index of starchy foods (Olkku and Rha, 1978; Collings
regarded as neutral or negative due to energy loss from drippings
et al., 1981; Snow and O'Dea, 1981; Tester and Sommerville,
or the chemical degradation of amino acids (Borenstein and
2000; Langkilde et al., 2002). Unfortunately, little empirical
Lachance, 1988). However, anthropologists have recently pro-
evidence exists that demonstrates the energetic benefits of
posed that there is an energetic benefit of cooking food and that the
cooking, especially the cooking of meat (Wrangham, 2006).
advent of cooking has had diverse effects on human evolution
High temperature has two major effects on the physical
(Wrangham and Conklin-Brittain, 2003). Suspected outcomes of
properties of meat (Bouton and Harris, 1972; Purslow, 2005;
the increase in energy efficiency from consuming cooked food
Tornberg, 2005). At around 40 °C, muscle fiber proteins start to
denature, leading to the contraction, drying and toughening of the
meat. Between 50 and 60 °C, collagen denatures, causing
? Corresponding author. Department of Biological Sciences, Box 870344,
gelatinization and solubilization of the connective tissues sheaths
University of Alabama, Tuscaloosa, AL 35487-0344, USA. Tel.: +1 205 348
surrounding the muscle fibers. As cooking proceeds the meat
1809; fax: +1 205 348 1786.
E-mail address: ssecor@biology.as.ua.edu (S.M. Secor).
becomes increasingly more tender and hence can be more easily
1095-6433/$ - see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.cbpa.2007.08.014

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S.M. Boback et al. / Comparative Biochemistry and Physiology, Part A 148 (2007) 651–656
chewed (Lucas, 2004). Meat quality and cooking method both
withheld food from pythons for a minimum of 30 days to ensure
determine the extent that heating impacts the shear values of the
that they were postabsorptive (Secor and Diamond, 1995).
meat and the perception of its tenderness (Harris and Shorthose,
1988; Combes et al., 2003; Ruiz de Huidobro et al., 2005).
2.2. Metabolic response to meal treatments
Conceivably, the denaturing of proteins and gelatinization of
collagen from cooking should facilitate the digestive actions of
For each metabolic trial, pythons (mean body mass ± 1
gastric acids and proteolytic enzymes (Davies et al., 1987).
SE = 569 ± 19 g, N = 16) were fed either an adult rat or one of
Hence, cooking should decrease the cost and time of gastric and
four experimental beef meals; intact raw beef, intact cooked beef,
intestinal performance, thereby increasing net energy gain.
ground raw beef, and ground cooked beef. Lean beef (eye of the
Similarly, the grinding of meat, akin to chewing, should also
round, less than ?5% fat) was purchased fresh for each trial from
decrease cost and increase gain. We tested the hypotheses that
a local meat supplier (South's Finest Meats, Tuscaloosa, AL,
cooking and grinding reduce the cost of meat digestion using the
USA) and cut to weigh 25% of the snake's body mass. For ground
Burmese python (Python molurus) as our experimental model
treatments, meat was ground using a manual meat grinder
(Secor and Diamond, 1998). Burmese pythons are strict
(Universal, L.F & C. New Britain, CT, USA). For cooked
carnivores and are able to ingest large intact meals that can
treatments, intact or ground raw meat was placed in a covered dish
exceed 50% of their body mass (Secor and Diamond, 1997).
and microwaved (Sharp R-409EW, Mahwah, NJ, USA) to an
After feeding, pythons experience a large metabolic response
internal temperature of 80 °C. Any water lost from the meat
[the specific dynamic action (SDA) of the meal], that is
during cooking was collected from the cooking dish and added to
equivalent to 28–37% of the ingested meal's energy (Secor
the meal when fed to the pythons such that there was no loss of
and Diamond, 1997). It is also known for the Burmese python
meal mass due to cooking. All snakes were carefully force fed
that reducing the structural integrity of their meal lowers SDA
meals either directly through their mouths (intact meals) or
(Secor, 2003). In this study we assessed the individual and
through a feeding tube (ground meals) (Fig. 1). We also fed eight
combined effects of the cooking and grinding of meat on python
pythons each a single adult rat equaling 25 ± 1% of their body
SDA.
mass to compare the metabolic responses of rat-fed pythons with
those of snakes fed the beef meals.
2. Materials and methods
We measured rates of oxygen consumption (V
? O2) of pythons
using closed-system respirometry (Vleck, 1987; Secor and
2.1. Study animals and their maintenance
Diamond, 1997). Fasted pythons were placed into individual
respirometry chambers (9-L plastic containers fitted with inflow
Hatchling Burmese pythons (P. molurus) (?120 g) were
and outflow stopcocks) and maintained within a temperature-
purchased commercially (Bob Clark Captive Bred Reptiles,
controlled environmental chamber at 30 °C. For each sampling
Oklahoma City, OK, USA; Strictly Reptiles Inc., Hollywood,
period, a 50-ml gas sample was drawn from each chamber, the
FL, USA) and maintained individually in 20-L plastic cages
chambers were sealed, and a second gas sample was drawn 1–2 h
within customized racks (Animal Plastics, Johnston, IA, USA).
later. Gas samples were injected into an O2 analyzer (S-3A/II, AEI
Embedded in the rear of each rack is a heat conductive cable that
Technologies, Naperville, IL, USA) after passing through a
maintains a rear to front gradient of 30–26 °C within each cage.
column of water absorbent (Drierite) and CO2 absorbent
Pythons were kept under a 14 h:10 h light:dark cycle, fed pre-
(Ascarite). We calculated V
? O2 (ml h?1) after correcting for
killed laboratory rodents every two weeks, and provided with
standard pressure and temperature as described by Vleck (1987).
water ad libitum. Prior to the start of metabolic experiments, we
We measured V
? O2 of fasted pythons at 12-h intervals for 4 days
Fig. 1. Force-feeding pythons intact cooked (A) and ground cooked meat (B).

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653
during digestion; 3) the factorial scope of peak V
? O2, peak V?O2
divided by SMR; 4) duration, the number of days that postfeeding
V
? O2 was significantly greater than SMR; 5) SDA, the summed
total energy expended above SMR during the postfeeding period
of significantly elevated V
? O2; and 6) SDA coefficient, SDA
quantified as a percentage of ingested energy. We quantified SDA
from the amount of extra O2 consumed above SMR during the
duration of significantly elevated V
? O2 and converted this value to
kilojoules assuming 19.8 J expended per ml O2 consumed
(Geesaman and Nagy, 1988).
2.4. Bomb calorimetry
We determined the energy content of each experimental meal by
multiplying its wet mass by its mass-specific energy value (kJ g? 1
wet mass) as determined by bomb calorimetry. For the rat meal,
eight rats (132 ± 5 g, range 117–161 g) were weighed, dried at
60 °C, reweighed, ground into a fine powder using an electric
grinder, and pressed into pellets. For each beef treatment, four 30–
70 g samples of meat were taken from four different pieces of beef,
and weighed, processed as above (intact raw, ground raw, intact
cooked, and ground cooked), dried at 60 °C, reweighed, ground
into a fine powder, and pressed into pellets. For each rat or meat
sample, three pellets were ignited in a bomb calorimeter (1266,
Parr Instruments Co., Moline, IL, USA) to determine energy
content. Wet mass energy content was calculated as the product of
dry mass energy equivalent as determined by bomb calorimetry,
and the sample's percent dry mass. Rats had an energetic
equivalent of 6.96 ± 0.11 kJ g? 1 wet mass. We found no significant
difference (P values N 0.28) in either the dry or wet mass energy
content among the four beef treatments and therefore elected to use
the wet mass energy value determined from the intact raw beef
samples (6.59 ± 0.14 kJ g? 1 wet mass) to calculate meal energy for
all beef treatments. Meal energy was therefore used to calculate the
SDA coefficients for each metabolic trial.
Fig. 2. Mean oxygen consumption (V
? O
2.5. Statistical analysis
2; ± 1 SE) of Burmese pythons (Python
molurus) prior to day 0 and up to 10 days after consuming meal treatments
equaling 25% of snake body mass (N = 8 for each meal treatment). Cooked
For each meal treatment we used a repeated-measures analysis
treatments (intact and ground) were obtained by microwaving lean (b5% fat)
of variance (ANOVA) to test for an effect of time (days
raw meat to an internal temperature of 80 °C. See Materials and methods for
postfeeding) on V
? O2. Each ANOVA was followed by a post hoc
details.
pairwise mean comparison to determine when postfeeding V
? O2
was no longer significantly different from SMR. To examine the
and assigned the lowest measure V
? O2 of each individual as its
effects of meal treatment, we tested for differences in body mass,
standard metabolic rate (SMR). The morning following SMR
meal mass, and metabolic measures among the four beef meals
measurements, snakes were removed from their respirometry
using ANOVA for body mass and mass-specific metabolic
chambers, fed their prepared meals, and returned to their chambers
measures, and ANCOVA (body mass as the covariate) for meal
for subsequent metabolic measurements. We measured V
? O2 of fed
mass and whole-animal metabolic measures. When effects of meal
snakes at 12-h intervals for 3 days and thereafter at 24-h intervals
treatment were significant, we used pairwise mean comparisons to
for 11 more days.
identify those treatments that differed significantly from each other.
For all statistical comparisons, we set ?=0.05 and report mean
2.3. Quantifying the cost of digestion
values as means ± 1 SE.
We examined the postfeeding metabolic response to digestion,
3. Results
absorption, and assimilation of each meal treatment by measuring
the following variables: 1) SMR, the minimum V
? O2 measured in
The digestion of the intact beef meal generated nearly
the 4 days prior to feeding; 2) peak V
? O2, the highest V?O2 measured
identical metabolic response to that stemming from the digestion

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Table 1
Body mass, meal mass, standard metabolic rate (SMR) and five variables of the metabolic response to digestion for Burmese pythons (Python molurus) digesting rats
and four beef meals
Variable
Rat
Intact raw beef
Intact cooked beef
Ground raw beef
Ground cooked beef
F
P
N
8
8
8
8
8
–
–
Body mass (g)
588 ± 31
572 ± 23
582 ± 18
586 ± 20
580 ± 43
0.04
0.987
Meal mass (g)
147 ± 8
143 ± 6
146 ± 5
147 ± 5
145 ± 11
1.12
0.359
SMR (ml h? 1)
16.5 ± 0.7
14.5 ± 0.9
15.0 ± 0.8
15.8 ± 0.9
16.3 ± 1.1
1.18
0.335
SMR (ml g? 1 h? 1)
0.028 ± 0.001
0.025 ± 0.001
0.026 ± 0.001
0.027 ± 0.001
0.028 ± 0.001
1.41
0.259
Peak V
? O2 (ml h?1)
217 ± 15
203 ± 13c
180 ± 10a,b
183 ± 6b,c
161 ± 10a
5.84
0.003
Peak V
? O2 (mL g?1 h?1)
0.379 ± 0.035
0.354 ± 0.013b
0.309 ± 0.015a
0.316 ± 0.012a
0.280 ± 0.009a
6.02
0.003
Scope of peak V
? O2
13.2 ± 0.7
14.0 ± 0.3c
12.1 ± 0.5b
11.9 ± 0.7b
10.0 ± 0.4a
10.4
0.0001
Duration (days)
8
8
7
7
7
–
–
SDA (kJ)
299 ± 17
303 ± 11c
262 ± 10b
263 ± 8b
232 ± 12a
9.14
0.0002
SDA (kJ kg? 1)
509 ± 23
532 ± 16b
452 ± 19a
452 ± 16a
411 ± 30a
5.72
0.0035
SDA coefficient
29.0 ± 1.33
32.3 ± 1.0b
27.4 ± 1.2a
27.4 ± 1.0a
24.9 ± 1.8a
5.72
0.0035
Values are presented as means ± 1 SE and F and P values result from either ANOVA or ANCOVA analysis of the four beef treatments. For variables with significant
ANOVA or ANCOVA, different supercript letters denote significant (P b 0.05) differences between the means as determined by post hoc pairwise comparisons (see text
for details).
of the more natural rat meal (Fig. 2). Both meals resulted in more
meals (12.1 ± 0.5) or the ground raw meals (11.9 ± 0.7), pythons
than a 13-fold increase in V
? O2, an 8 day duration of elevated
possessed significantly (P values b 0.014) higher scopes com-
metabolism, and very similar SDA (Table 1). Given this match in
pared to when digesting the ground cooked meals (10.0 ± 0.4)
the metabolic responses between the rat and intact raw beef
(Table 1). For the intact raw meals, pythons maintained elevated
meals, we focused our remaining comparisons on the four beef
rates of metabolism for 8 days, whereas for the other three meal
treatments.
treatments, metabolism was elevated for seven days (Table 1).
Python body mass, meal mass, and SMR (mass-specific and
The total energy expended on digestion, absorption, and
whole animal) did not significantly differ among the four beef
assimilation (SDA), quantified as kJ, kJ kg? 1, and as a percentage
treatments (Table 1). For each treatment, V
? O2 of pythons had
of meal energy, differed significantly (P values b 0.004) among
increased significantly (P values b 0.0002) within 12 h after
meal treatments (Table 1). The consumption of the intact raw
feeding, peaked between 1.5–2.5 days postfeeding, and remained
meals resulted in a significantly greater SDA (303 ± 11 kJ)
significantly elevated above SMR for a total of 7 or 8 days
compared to that generated by the other three meals. Both the
(Fig. 2). Both the postprandial peak in V
? O2 and the factorial scope
whole cooked (262 ± 10 kJ) and ground raw (263 ± 8 kJ) meals
of peak V
? O2 varied significantly (P valuesb0.004) among
generated significantly greater SDA compared to the ground
treatments (Table 1). We found that meals of intact raw beef
cooked meal (232 ± 12 kJ). This pattern was repeated with the
generated significantly (P values b 0.035) higher peaks in V?O2
analysis of mass-specific SDA (kJ kg? 1; Table 1). When
(203 ± 13 ml h? 1) compared to both whole (180 ± 10 ml h? 1) and
calculated as a percentage of meal energy, SDA was significantly
ground (161 ± 10 ml h? 1) cooked meals. In addition, ground raw
(P = 0.002) higher for the intact raw meal (32.3 ± 1.0%) compared
meals expressed a significantly (P = 0.042) higher V
? O2 peak
to the ground cooked meal (24.9 ± 1.8%; Table 1).
compared to ground cooked meals (Table 1). Correspondingly,
the factorial scope of peak V
? O2 was significantly greater
4. Discussion
(P values b 0.013) for the intact raw meals (14.0 ± 0.3) compared
to the other three beef meals. When digesting the intact cooked
We found using Burmese pythons that the cooking of meat
significantly reduces the combined costs of its digestion and
assimilation, the meal's SDA. When the intact or ground meat
was microwaved, the SDA fell by an average of 12.7%.
Likewise, the grinding of meat (raw or cooked) reduced SDA by
an average of 12.4%. Together, cooking and grinding lowered
SDA by 23.4% (Fig. 3). These results demonstrate that cooking
and grinding separately, and in combination, decrease the
amount of energy required to digest and assimilate a meal. The
similarities in the decrease in SDA due to cooking and grinding
and their additive effects suggest that the cooking of meat has as
much impact on facilitating digestion as grinding or chewing.
Since snakes do not masticate their food, the stomach alone
must break down each meal into a soup-like chyme suitable for
passage into the small intestine (Secor, 2003). If the integrity of
Fig. 3. The effect of grinding and cooking1 meat on the energy needed to digest,
their intact meal is compromised by grinding then some of the
absorb, and assimilate meat meals in pythons. Bars are scaled as a percentage of
intact raw meat.
mechanical and chemical digestion normally performed by the

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655
stomach has already been accomplished, thus decreasing gastric
these studies will be instrumental in demonstrating the impact of
effort. This was observed when pythons were fed homogenized
cooking on human energetics and its role in the evolution of
rats and the resulting SDA was 75% of that generated by the
human life history.
digestion of intact rats (Secor, 2003). Likewise we found in this
study that the grinding of meat significantly reduces SDA.
Acknowledgements
Compared to the raw meals, the cooked meals took less time
(for intact meals) and energy (for intact and ground meals) to
We would like to thank M. Addington, K. Asbill, and E.
digest and assimilate. Cooking softens and solubilizes the
Newsom for assistance with python care and metabolic
collagen-rich connective tissues that surround the muscle fibers
experiments. This project was supported in part by a grant
thereby increasing access of the tissue to gastric acids and
from the National Science Foundation (IOB-0466139) to SMS.
proteolytic enzymes. Therefore like grinding, cooking reduces
This research was approved by the University of Alabama
the structural integrity of the meat, thereby speeding up gastric
Institutional Animal Care and Use Committee.
digestion.
As implied, we suspect that the reduction in SDA for ground
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